Information recorded on compact disks (CDs), digital versatile disks (DVDs) or other types of optical disks can be read out by casting a laser beam through the readout surface onto the information layer and detecting the beam reflected by the information layer. Therefore, if a scratch is present on the readout surface, the information cannot be correctly read out because the readout beam is scattered by the scratch.
However, presence of a scratch on the readout surface does not damage the information itself because the actual information is not recorded on the readout surface. Accordingly, the readout surface is polished to remove the scratch and thereby restore the optical disk so that the information can be read out again. Usually, this polishing process is continued until the readout surface becomes as smooth as a mirror surface.
Conventional processes for polishing optical disks can be categorized into two types: dry process and wet process.
A dry polishing is a process of polishing an optical disk in the air, using a polishing liquid (e.g. a liquid compound) and a polishing body (called a “buff”) made of a cloth, felt, sponge or similar materials. By the dry polishing process, removing deep scratches are impossible, or time-consuming even if it is possible. Therefore, it is necessary to perform a rough sanding process using abrasives or a similar sanding body before the mirror-polishing process. However, since the rough sanding process produces a frictional heat or causes the clogging of the sanding body with shavings, it is still difficult to remove deep scratches in a short period of time.
Accordingly, in recent years, wet polishing processes have become more popular. In the wet polishing process, water containing a surfactant or similar chemicals (called “sanding agent” hereinafter) is supplied on the interface between the optical disk and the polishing body. The use of the sanding agent effectively reduces the frictional heat during the sanding process and removes the shavings from the interface, so that even a deep scratch can be removed in a short period of time.
In the wet polishing process using the sanding agent, the resulting wastewater contains impurities, such as the polishing liquid, shavings and abrasive grains have come off the polishing body. Therefore, in most cases, the wastewater is disposed of, without being recycled.
The wastewater containing the aforementioned impurities must be disposed of as industrial waste. However, its disposal is not so easy now since the restriction on the disposal of industrial waste has been tightened. Furthermore, in some areas where tap water is hard water (e.g. Okinawa in Japan, Europe or China), it is not cost-effective to dispose of the wastewater that has been used only once, because soft water must be costly purchased for use in the sanding agent.
Accordingly, for the purpose of recycling the wastewater, an apparatus for removing the impurities from the wastewater with a filter has been introduced. In such an apparatus, the filter needs to have a mesh size of about 1 μm to remove polishing particles having a diameter of 5 μm or smaller, which originate from the polishing liquid and are finally discharged with the wastewater. This requirement leads to the problem that the filter is easily clogged in a short period of time if the wastewater is filtered without undergoing any pre-processing.
One solution to prevent the clogging of the filter is to treat the wastewater so that the impurities contained in it are cohered into larger grains (or ‘flocs’) that can be removed with a filter having a larger mesh size. A known method for creating flocs of imputes uses a flocculant.
However, this approach has some problems. Firstly, it requires some operations, including the steps of controlling the amount of the flocculant according to the amount of the wastewater, stirring the wastewater after the flocculant is added, and leaving the stirred wastewater untouched for some time. Secondly, this approach needs at least two separate tanks, one for holding the wastewater and another for holding the flocculant. This construction makes the apparatus larger. Finally, the surfactant added to the wastewater may impede the flocculation process, or a portion of the flocculant may absorb the surfactant.
A more advantageous method for creating flocs of impurities uses electrolysis. In this method, which is called the electroflocculation, aluminum ions are eluted from an aluminum electrode during the electrolytic process, and the released ions react with the hydroxyl ions to produce aluminum hydroxide. The flocculating action of the aluminum hydroxide creates flocs of the impurities.
There are some documents disclosing water purification apparatuses based on the above-described principle. For example, Patent Document 1 discloses a device for purifying water discharged from a bathtub, using the electrolysis of aluminum. Patent Document 2 discloses a water cleansing method in which the electrolysis of wastewater using aluminum and iron plates is combined with a membrane separation technique. Patent Document 3 discloses an electrolysis type turbid water treatment apparatus using an auxiliary electrode consisting of small chips of aluminum obtained from used cans.
[Patent Document 1] Unexamined Japanese Patent Publication No. H08-132051 (Japanese Patent No. 2871490), “Water Purifying Device”, Paragraph 0021 and FIG. 1 etc.
[Patent Document 2] Unexamined Japanese Patent Publication No. 2001-157894, “Water Cleansing Method”
[Patent Document 3] Unexamined Japanese Patent Publication No. 2004-66010, “Electrolysis Type Turbid Water Treatment Apparatus”